<div dir="ltr"><div class="gmail_extra"><div class="gmail_quote">On Mon, Oct 14, 2013 at 2:02 AM, Marius Schrecker <span dir="ltr"><<a href="mailto:marius.schrecker@lyse.net" target="_blank">marius.schrecker@lyse.net</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="HOEnZb"><div class="h5"><br>On Monday, October 14, 2013 10:42 CEST, Stephen Worthington <<a href="mailto:stephen_agent@jsw.gen.nz" target="_blank">stephen_agent@jsw.gen.nz</a>> wrote:<br>
<blockquote><br><br>Just a few thoughts of things that have bitten me in the past.<br><br>There is serious potential for ground loops - is the PC on a basement<br>power circuit? And are the USB hub and the amplifier the audio cables<br>
plug into plugged into the sitting room mains? Combine that with the<br>earth paths through the aerials into the tuners, and you could be<br>picking up a lot of hum. And there could even be serious voltage<br>differences between the basement and sitting room circuit's earths.<br>
Put a meter between an earth coming up from the basement and the<br>sitting room earth to check for that.<br><br>Long cables are also aerials, and can pick up all sorts of strange<br>signals when not properly shielded - not just from nearby cables or<br>
equipment, but any transmitter they are the right wavelength to<br>
receive. Powerful nearby radio stations are easy to receive on a long<br>cable of a matching wavelength and their signal can swamp other<br>signals.<br><br>My (tiny) experience with a long USB cable is that the voltage drop on<br>
one is severe - you need to use cables with voltage regenerators to<br>get the device at the far end with enough voltage to work properly. Or<br>use a self-powered device at the far end, or a powered hub. Of<br>course, there are hubs and hubs - some USB devices do not like to be<br>
on any hub, and some hubs work much better than others.<br><br>The imon display presumably has a backlight, so is drawing a fair bit<br>of power. So check the voltage it is receiving at the end of its<br>ISDN/RJ45 power cable while it is drawing that power - it might be<br>
enough for the backlight, but not enough for its USB transmitter to<br>pump out a valid signal back through a long cable.<br><br>Problems like this are usually fixable
if you know what the cause is -<br>so you may need to be using an oscilloscope to look at your signals to<br>diagnose the problem.<br>_______________________________________________<br>mythtv-users mailing list<br><a href="mailto:mythtv-users@mythtv.org" target="_blank">mythtv-users@mythtv.org</a><br>
<a href="http://www.mythtv.org/mailman/listinfo/mythtv-users" target="_blank">http://www.mythtv.org/mailman/listinfo/mythtv-users</a></blockquote><br><br></div></div>Thanks Stephen,<br><br> Very good pointers to what to look at.<br>
<br>What I hadn't calculated at all was that the computer and the power to the rest of the system ARE on separate mains circuits now - deliberate to minimise the risk of power being cut to the computer - and I didn't think about potential problems with earth loops. However, the only earth to earth connection is over hdmi (to the TV). Good point with the cables acting as antennę. I think I should certainly be looking at good quality, double shielded cables of a minimum length for purpose. Also possibly swapping out the isdn cable for good quality cat5-6 mi
ght help with pd drop to the imon lcd.<br><br> I'll start by trying the same cables but in free air, to rule out conduiting problems, then shorten one cable at a time to see which one is having problems with the extra length. I might also try TWO posered USB hubs, one at each end of the usb connection, acting as repeaters so that I get the strongest possible signal through the cable.<br>
<br>Thabnks again for your input!<span class="HOEnZb"><font color="#888888"><br><br>Marius</font></span><br></blockquote></div><br></div><div class="gmail_extra">I was going to suggest trying high-quality cables, but you've got that covered. Also, USB hubs are not all the same. If you're going to buy another one, try a different brand/model and first try it in place of the existing one.<br>
<br>Karl<br></div></div>